1. Field of the Invention
The present invention relates to a communication system between a host station and a substation which preferably includes a non-contact tag device. More particularly, the present invention relates to a communication system for implementing processing such as data communication by identifying a plurality of moving substations.
2. Description of the Related Art
Conventionally, communication between a host station and a substation utilizing a weak radio wave has been made by controlling them in a 1-to-1 relationship and by polling the substation from the host station. However, when a plurality of substations exist, there has been a problem that responses to the polling from the host station made from the substations conflict and thereby no communication can be made.
Then, as a method for solving such a problem, a communication method disclosed in Japanese Patent Publication No. Hei. 3-2271, for example has been proposed. According to this method, when a monitor station, i.e., a host station, issues an inquiry signal containing a monitor station ID code and when a tag circuit, i.e., a substation, generates a response signal containing a tag circuit ID code in response to the inquiry signal, the response signal generated by the tag circuit is input at random to one time slot among a plurality of time slots provided in the monitor station.
However, the prior art method described above has a problem in that although it provides a possibility that a plurality of tags may be simultaneously processed, a probability of a communication failure is not negligible. That is, because a plurality of response signals of the tags are input while each tag selecting its time slot at random, there is a possibility that the same time slot is selected for the substations, thus causing a conflict among the response signals. The probability of the conflict is (1/number of time slots). Although this conflicting probability may be reduced of course by re-transmitting the response signal, the probability cannot be reduced to zero by a finite number of times of re-transmission.
As a countermeasure, a method of increasing a number of time slots or of increasing a number of times of re-transmission is conceivable and may in fact allow reduction of the probability of a communication failure due to the conflicts to a negligible degree. However, when the tag is attached to an object moving at high speed for example, a sufficient number of time slots or sufficient number of times of re-transmission may not be assured because a communication window time between the host station and the substation is limited.